Atmospheric circulation over the Bolivian Altiplano during ... - CDAM

1598M. VUILLEand Salta, no change over La Paz and a decrease over Lima). The fact that this anticyclone is intensifiedand centred over the southern Altiplano, south of its climatological position, during rainy periods, isfurther corroborated by the analysis of monthly NCEP reanalysis data and consistent with similar resultsreported by Aceituno and Montecinos (1993). Besides the upper-air divergent outflow related to the highpressure system, upper-air easterly winds to the north of the anticyclone lead to a turbulent entrainmentof near-surface winds, accelerating moisture transport in lower levels towards the Altiplano (Garreaud,1999). Often the upper-air high pressure system is located off the Pacific coast, to the southwest of theAltiplano, enhancing the southeasterly flow and upper-air divergence over the Altiplano (Jacobeit, 1992;Vuille et al., 1998).Circulation mechanisms during WET periods in winter (JJA) are entirely different from in the summer,yet very little has been published to date about atmospheric circulation during WET periods on theAltiplano in austral winter. This period is generally believed to be entirely dry. However, recent snowaccumulation measurements on the summit of Nevado Sajama yielded 53 cm of snowfall from Julythrough September 1997 (Hardy et al., 1998), and Vuille and Baumgartner (1998) reported an average ofthree to five snowfall events per winter in the southern Altiplano. Vuille and Ammann (1997), analysingwinter snowfall events on the Altiplano between 1984 and 1993 using NOAA/AVHRR and GOESsatellite data, ECMWF gridded upper-air data and radiosonde data from Antofagasta, have shown thatprecipitation between May and September is associated with outbursts or northward intrusions of polarair masses from the planetary west wind zone. Cold fronts, or upper-air low pressure systems (cut-offlows), that penetrate into the southern Altiplano trigger the rare winter precipitation events on NevadoSajama. The radiosonde analysis presented here confirm these results, revealing increased northerly andwesterly wind components, reduced pressure and temperatures, and increased specific humidity over theentire Altiplano during such events. The fact that these anomalies are more pronounced over the southernand western stations Antofagasta, Lima and Salta than over La Paz provide clear evidence for the Pacificorigin of these events.The results from this analysis of atmospheric circulation during WET and DRY periods over the SouthAmerican Altiplano in austral summer (DJF) and winter (JJA) have clear implications for the interpretationof the proxy data obtained from ice cores from Sajama summit. Most importantly they show thatprecipitation events on Sajama are related to large-scale circulation changes over the entire Altiplanoregion, and can be traced in radiosonde data to the east and the west of the Central Andes. Accordingly,proxy data from the ice core are of more than just local significance, and can be linked to circulationfeatures on a larger scale. Furthermore, Nevado Sajama lies in an area which is influenced by two entirelydifferent circulation regimes during the course of the year. Due to the seasonal shifts of the circulationzones, it experiences tropical as well as extra-tropical precipitation events. This is of importance becausepast climatic changes, recorded in the ice cores, might have been caused by shifts in circulation zones. Inthese terms, Nevado Sajama is an excellent place for palaeoclimatic studies.The El Niño–Southern Oscillation phenomenon has a significant impact on the Altiplano climate,especially during austral summer. Atmospheric circulation anomalies however are more significant thanchanges in precipitation, which are statistically insignificant in the Nevado Sajama region. Nonetheless,precipitation tends to be deficient during LOW index summers (El Niño), and above average duringHIGH (La Niña) and LOW+1 (DJF following El Niño) summers. The tendency towards increasedaridity during LOW summers can at least partially be attributed to a higher zonal wind component(westerly wind anomalies) in the middle and upper troposphere, preventing penetration of moist airmasses from the eastern interior of the continent into the Sajama area. These westerlies are associatedwith a northward displaced and weakened Bolivian High. In this sense LOW index periods arecharacterized by similar anomalies as DRY periods, and can be regarded as an extended DRY period ora summer with an increased occurrence of DRY episodes. Furthermore, both LOW and DRY summersfeature significantly above average upper-air temperatures (200 hPa). Although not always significant atall pressure levels, the general state of the atmosphere over the Altiplano during austral summer LOWindex phases can be characterized by an increased westerly and northerly wind component, reducedspecific humidity, an increase in atmospheric temperatures and a vertically expanded troposphere.Copyright © 1999 Royal Meteorological Society Int. J. Climatol. 19: 1579–1600 (1999)